> On Sun, Feb 04, 2007 at 03:12:32PM -0500, Bill Davidsen wrote:> > Arjan van de Ven wrote:> > >>Because user threading can avoid context switches, there will always be > > >>cases where it will outperform o/s threads for hardware reasons.> > >> > >actually.. switching from one "real" thread to another in Linux is not> > >an actual context switch in the hardware sense... at least this part of> > >your argument seems to be incorrect ;)> > >> > How does that work? Switching between kernel threads requires going into > > the kernel, user level thread switches are all done in user mode.> > > > Do you have some way to change o/s threads w/o going into the kernel?> > But going into kernel is not very expensive on Linux.> > On the other side, the overhead you need to add for every single syscall> that might block for the M:N threads and the associated complications> which make it far harder to conform to POSIX IMHO far outweight the costs> of going into the kernel for a context switch.

Agreed, definitely. A libpcl (using swapcontext(3)) cobench is about 50 times faster than an context switch measured by lmbench (although I have serious doubts about about the ability of lat_ctx to measure it - but that's another story) on an Opteron 254. One may say "Wow! Really?!?".The point is, who cares. We are talking about differences between super-fast (~2us) and ultra-fast (~0.04us).The time (and code) that you'll have to drop in the syscall path to handle M:N is very likely to make you lose way more of what you gain by avoiding an OS context switch (a "soft" context switch you still have to do it).Either use N:N (requires locking, but spread over multiple CPUs) or 1:N (I/O driven state machines or coroutines - no locking, once-CPU bound).